Small particles, including microspheres and nanospheres, have many medical uses in diagnostic and/or therapeutic procedures. In selected clinical applications, it may be advantageous to provide such microspheres and nanospheres with physical and/or chemical qualities to enhance their visual identification using various clinical imaging modalities to make the in vivo location of such microspheres apparent to a user. Such microspheres and nanospheres may allow for selective embolization of certain sized blood vessels under visual control using various clinical imaging modalities in specific clinical uses.
Most particles used in medical applications are characterized by numerous disadvantages including irritation of the tissues with which they come in contact and initiation of adverse immune reactions. Additionally, many of the materials used to prepare these particles may degrade relatively rapidly within the mammalian body, thereby detracting from their utility in certain procedures where long term presence of intact particles may be necessary. Moreover, the degradation of these materials may release toxic or irritating compounds causing adverse reactions in the patients.
Some known particle types suffer from difficulties in achieving desirable suspension properties when such particles are incorporated into a delivery suspension for injection into a site in the body to be treated. Many times, the particles settle out or tend to “float” in the solution such that they are not uniformly suspended for even delivery. Furthermore, particles may tend to aggregate within the delivery solution and/or adhere to some part of the delivery device, making it necessary to compensate for these adhesive/attractive forces.
In order to achieve a stable dispersion, suitable dispersing agents may be added, which may include surfactants directed at breaking down attractive particle interactions. Depending on the nature of the particle interaction, materials such as the following may be used: cationic, anionic, or nonionic surfactants such as Tween™ 20, Tween™ 40, Tween™ 80, polyethylene glycols, sodium dodecyl sulfate, various naturally occurring proteins such as serum albumin, or any other macromolecular surfactants in the delivery formulation. Furthermore thickening agents can be used help prevent particles from settling by sedimentation and to increase solution viscosity, for example, polyvinyl alcohols, polyvinyl pyrrolidones, sugars, or dextrins. Density additives may also be used to achieve buoyancy.
It can also be difficult to visualize microparticles in solution to determine their degree of suspension when using clear, transparent polymeric acrylate hydrogel beads in aqueous suspension. The inert precipitate barium sulfate may be used in particle form an additive for bone cement, for silicones, for rendering items visible during X-ray examination and for providing radiopacity to polymeric acrylate particles. See Jayakrishnan et al., Bull. Mat. Sci., Vol. 12, No. 1, pp. 17-25 (1989). Barium sulfate also is known for improving fluidization and is often used as an inorganic filler to impart anti-stick behavior to moist, aggregated particles. Other attempts to increase visualization of microparticles include the use of gold, for example, in Embosphere Gold™, which provides a magenta color to acrylate microparticles using small amounts of gold.
In certain medical applications, it may be of further value to provide microparticles such as microspheres in one or more sizes. Furthermore, it may also be of value to provide each of such sizes of microspheres incorporated with color-coded associated dyes to indicate the microsphere size to the user. In yet other applications of use, it may further be of value to provide sized and color-coded microspheres to a user in similarly color-coded syringes or other containers for transport and delivery to further aid a user in identifying the size of microspheres being used.
There thus exists in the art a need for small particles that can be formed to have a preferential generally spherical configuration for certain applications such as various therapeutic and diagnostic procedures which are not degraded by the natural systems of the mammalian system, are biocompatible, are easy to visualize in suspension while in use and/or demonstrate acceptable physical and suspension properties.